Does anyone know why the wheel won't come off when I've removed the 4 lug bolts?

 

likely rust is firmly attaching it ... hard impact at the top/bottom to loosen , when you remount clean the inner hub and the wheel with a wire brush and apply a light coat of never seize to prevent it from happening again,,,

If it's really really stuck , I've loosened the nuts a turn or two and drive it a few feet to loosen it